Typical melt spinning polymers, such as polyolefins, tend to be in a semi-crystalline state upon meltblown fiber extrusion (as measured by differential scanning calorimetry (DSC)). For polyolefins, this ordered state is due, in part, to a relatively high rate of crystallization and to the extensional polymer chains orientation in the extrudate. In meltblown extrusion, extensional orientation is accomplished with high velocity, heated air in the elongational field. Extending polymer chains from the preferred random coiled configuration and crystal formation imparts internal stresses to the polymer. Provided the polymer is above its glass transition temperature (T.sub.g) these stresses will dissipate. For meltblown polyolefins, the dissipation of stresses occurs spontaneously since the polymer's T.sub.g is well below room temperature.
In contrast, some melt spinning polymers, such as polyethylene terephthalate (PET), tend to be in a nearly completely amorphous state upon meltblown fiber extrusion. This characteristic is attributable to a relatively low rate of crystallization, a relatively high melt temperature (T.sub.m), and a T.sub.g well above room temperature. The internal stresses from amorphous orientation within the elongational field are frozen-in due to rapid quenching of the melt, thus preventing relaxation which cannot be released until subsequent annealing above T.sub.g. Annealing between T.sub.g and the T.sub.m for sufficient periods allows the polymer to both crystallize and dissipate internal stresses caused by elongational orientation. This stress dissipation manifests itself in the form of shrinkage that can approach values exceeding 50% of the web's extruded dimensions.
The textile and film industries have successfully addressed dimensional instability in woven polyester fabrics and films using edge tentering during heatsetting or annealing. In edge tentering, the woven polyester fabric or film is held along its edges to a desired width as it passes through an annealing oven. The heatsetting temperature ranges typically from about 177.degree. C. to about 246.degree. C. (350.degree. F. to about 475.degree. F.), and the dwell time ranges from about 30 seconds to several minutes. The annealed article is dimensionally stable up to the heatsetting temperature. While edge tentering is practical for films and woven fabrics, nonwoven fibrous webs typically lack sufficient tensile properties (i.e., fiber and web strength) to withstand conventional edge tentering procedures, resulting in a damaged web.
Various attempts have been made in the art to achieve a dimensionally stable polyester nonwoven fibrous web. U.S. Pat. No. 3,823,210 (Hikaru Shii et al.) describes a method of manufacturing an oriented product of a synthetic crystalline polymer. The patent discloses drawing a crystalline polymer, applying tensile stress in the direction of the draw axis in a heated solvent, and under this condition extracting the soluble fractions of the drawn material.
U.S. Pat. No. 5,010,165 (Pruett et al.) describes a dimensionally stable polyester melt blown web achieved by treating a melt blown web composition with a solvent where the solvent has a certain solubility parameter, and drying the melt blown web composition.
U.S. Pat. No. 5,364,694 (Okada et al.) teaches that PET cannot give a meltblown web with small thermal shrinkage unless the melt-blowing operation is conducted at higher viscosity and with air under higher pressure than these melt-blowing conditions employed for other readily-crystalline polymers such as polypropylene. The patent teaches stable operation with high productivity is impossible under such strict conditions. The patent discloses that blending the PET with 2 to 25% of a polyolefin decreases the melt viscosity of the entire blend so that the polymer extrudates can be attenuated into fibers even by the comparatively weak force exerted by a low-pressure air of not more than 1.0 kg/cm.sup.2. The extruded polyolefin has a high crystallization rate. In the blend, the polyolefin forms minute islands in a continuous sea of PET. The multiplicity of crystallized polyolefin islands constitute restricting points that suppress movement of amorphous molecules of PET when the web is heated, thereby preventing the nonwoven fabric from shrinking to a large extent.
U.S. Pat. No. 5,609,808 (Joest et al.) describes a method of making a fleece or mat of filaments of a thermoplastic polymer having both a crystalline and an amorphous state. A melt-blowing head is operated under conditions to produce long filaments, which are collected on a sieve belt and form crossing welds at cross-over points. The resulting web is composed of filaments having a diameter of less than 100 micrometers and a degree of crystallinity of less than 45%. The web is heated to a stretching temperature of 80.degree. C. to 150.degree. C. and is then biaxially stretched by 100% to 400% before being thermally fixed at a higher temperature. The stretching station can have a downstream pair of rolls which are driven at a certain speed and an upstream pair of rolls driven at a higher speed to effect the longitudinal stretching. Transverse stretching is effected between pairs of diverging chains.